Reproduction and DNAAQA GCSE Biology: Revision notes
Section 1
What are the advantages and disadvantages of sexual and asexual reproduction?
Sexual reproduction involves two parents and the fusion of gametes (sex cells), whilst asexual reproduction involves a single parent and produces offspring without gamete fusion.
| Aspect | Sexual Reproduction | Asexual Reproduction |
|---|---|---|
| Number of parents | Two | One |
| Genetic variation | High – offspring are genetically different | None – offspring are clones |
| Speed of reproduction | Slower (requires finding a mate, meiosis, fertilisation) | Faster (mitosis only) |
| Energy cost | High (courtship, gamete production) | Low (only requires mitosis) |
| Adaptation to environment | Better – variation allows population to adapt to change | Poorer – population cannot adapt if environment changes |
| Survival advantage | Lower individual survival (half genes from each parent) | Higher individual survival in stable conditions |
Key advantages of sexual reproduction: Produces genetic variation; allows populations to evolve and adapt to environmental change; increases chance of species survival.
Key disadvantages of sexual reproduction: Time-consuming; energetically expensive; requires two parents; produces slower population growth.
Key advantages of asexual reproduction: Fast reproduction; low energy cost; single parent needed; all offspring survive if parent is well-adapted.
Key disadvantages of asexual reproduction: No genetic variation; population cannot evolve; vulnerable to disease; poor adaptation to environmental change.
Examiners want you to link advantages/disadvantages to real-world situations: mention that sexual reproduction aids evolution and species survival, whilst asexual is efficient for rapid colonisation in stable environments.
Think of sexual reproduction as creating a varied team with different strengths (adaptability), while asexual reproduction is cloning a perfect worker for today's job – brilliant now, but vulnerable if conditions change.
Section 2
Why does sexual reproduction cause genetic variation whilst asexual reproduction produces genetically identical offspring?
Sexual reproduction produces genetic variation because:
- Meiosis shuffles chromosomes – Homologous chromosomes randomly separate during meiosis, creating gametes with different combinations of alleles
- Fertilisation combines genetic material from two parents – Each parent contributes different alleles, so offspring inherit a unique mix
- Crossing over – During meiosis, homologous chromosomes exchange segments of DNA, further increasing variation
Result: Every offspring is genetically different from parents and siblings (except identical twins).
Asexual reproduction produces genetically identical offspring because:
- Only mitosis is involved – Mitosis produces exact copies of the parent cell's chromosomes
- No gamete fusion – A single parent contributes 100% of the genetic material
- No shuffling of chromosomes – Daughter cells are clones with identical DNA sequences
Result: Offspring are genetically identical to the parent and to each other (barring rare mutations).
Key difference: Sexual reproduction mixes genes from two different sources via meiosis and fertilisation; asexual reproduction copies genes unchanged via mitosis.
Students often say asexual reproduction has 'no variation' – be precise: there is no genetic variation (mutations are rare), but environmental factors may still cause phenotypic differences.
Humans reproduce sexually: your meiosis shuffled your mother's and father's chromosomes differently than in your sibling, so you inherit different combinations of alleles. A bacterium reproduces asexually: it copies its entire chromosome by mitosis, creating a clone with identical genes.
Section 3
What is the structure of DNA and how are nucleotides organised?
DNA (deoxyribonucleic acid) is a double helix – two strands of nucleotides twisted together in a spiral shape.
Structure of a nucleotide:
Each nucleotide consists of three components:
- Sugar – deoxyribose (a five-carbon sugar)
- Phosphate group – links nucleotides together in a chain
- Nitrogenous base – one of four types: Adenine (A), Thymine (T), Cytosine (C), or Guanine (G)
How nucleotides are joined:
- Sugar-phosphate backbone – The sugar of one nucleotide bonds to the phosphate group of the next, forming the backbone of each strand
- Complementary base pairing – Bases on opposite strands bond together: A pairs with T (two hydrogen bonds); C pairs with G (three hydrogen bonds)
- Double helix – The two strands twist around each other, held together by hydrogen bonds between complementary bases
Key structural features:
- The two strands run in opposite directions (antiparallel)
- The structure is stable due to hydrogen bonds
- Complementary base pairing ensures the two strands are complementary copies of each other
When describing DNA structure, examiners expect you to mention: sugar-phosphate backbone, complementary base pairing, double helix, and hydrogen bonds. Always link the structure to function – the shape protects bases inside and allows easy replication.
Section 4
How do genes code for proteins and what is the genetic code?
A gene is a sequence of DNA bases (typically 1000+ bases) that codes for a specific sequence of amino acids, which are joined together to form a protein.
The genetic code:
- The genetic code is a triplet code – every three consecutive bases (called a codon) code for one amino acid
- There are 64 possible codons but only 20 amino acids, so the code is degenerate (multiple codons can code for the same amino acid)
- The code is universal – almost all organisms use the same genetic code
- One codon is the start codon (ATG) which initiates protein synthesis
- Three codons are stop codons which signal the end of protein synthesis
Example:
DNA sequence: ATG-GCT-TAC-CAA-TGA (nucleotides grouped in threes)
Codons are read on mRNA (so DNA is transcribed first):
mRNA sequence: AUG-GCU-UAC-CAA-UGA
Amino acids: Methionine-Alanine-Tyrosine-Glutamine-STOP
Thus, this gene codes for a protein made of four amino acids.
Why the triplet code?
- Four bases taken in groups of 2 would give only 16 combinations (not enough for 20 amino acids)
- Groups of 3 give 64 combinations (sufficient for all amino acids plus stop signals)
If a gene has the DNA sequence AAATTTCCC, it contains three codons. When transcribed to mRNA (AAAUUUCCC), this codes for three amino acids. Changing even one base can alter the amino acid sequence and protein function – this is a gene mutation.
Section 5
How is a protein made from a gene? (Transcription and Translation)
Protein synthesis occurs in two stages: transcription and translation. [Higher Tier]
Stage 1: Transcription (in the nucleus)
DNA is transcribed into messenger RNA (mRNA):
- RNA polymerase binds to the promoter region of a gene
- The enzyme unwinds the DNA double helix
- Free RNA nucleotides pair with complementary DNA bases (using A, U, C, G; note: uracil (U) replaces thymine in RNA)
- The mRNA strand is assembled: A on DNA pairs with U on mRNA; T pairs with A; C pairs with G; G pairs with C
- mRNA is released and travels from the nucleus to the ribosome in the cytoplasm
Key difference from DNA replication: Only one strand of DNA is transcribed (the template strand); mRNA is temporary.
Stage 2: Translation (at the ribosome)
mRNA is translated into a protein:
- mRNA binds to a ribosome in the cytoplasm
- tRNA molecules (transfer RNA) bring amino acids to the ribosome
- Each tRNA has an anticodon that matches the codon on mRNA (complementary base pairing rules apply)
- Amino acids are joined together by peptide bonds in the order specified by mRNA codons
- When a stop codon is reached, the protein is released
In summary:
DNA (nucleus) → Transcription → mRNA (cytoplasm) → Translation → Protein (at ribosome)
Or: Gene → mRNA → Protein
Examiners test understanding of both stages separately. For transcription, emphasise: DNA unwinds, mRNA is synthesised using complementary base pairing (with U instead of T), and mRNA leaves the nucleus. For translation, stress: tRNA anticodons match mRNA codons, amino acids are assembled in order, and stop codons terminate synthesis.
Students often confuse the direction or forget that mRNA is temporary. Remember: transcription makes mRNA (which is then degraded); translation uses mRNA to make a permanent protein. Also, tRNA brings amino acids – it is not the template.
Section 6
What is the human genome project and why is it important?
The Human Genome Project (HGP) was an international scientific effort to identify and map all human genes and determine the complete sequence of DNA bases in the human genome (approximately 3 billion base pairs).
Key facts about the HGP:
- Started: 1990
- Completed: 2003
- Scope: Identified roughly 20,000–25,000 human genes and sequenced the entire human genome
- Outcome: Created a complete reference map of human DNA
Potential benefits of the human genome project:
| Benefit Area | Details |
|---|---|
| Medical diagnosis | Identify genes responsible for genetic diseases (e.g. cystic fibrosis, sickle cell anaemia, Huntington's) |
| Disease treatment | Develop targeted therapies and gene therapy for inherited conditions |
| Personalised medicine | Tailor treatments based on individual genetic makeup |
| Drug development | Design drugs that target specific genes or proteins |
| Understanding evolution | Compare human DNA with other species to trace evolutionary relationships |
| Cancer research | Identify oncogenes (cancer-causing genes) and develop better treatments |
| Prenatal screening | Test foetuses for genetic abnormalities during pregnancy |
| Insurance and employment | Potential to predict disease risk (also raises ethical concerns) |
Why it matters:
Knowledge of the human genome has revolutionised medicine and biology, enabling early detection of diseases, development of gene therapies, and a deeper understanding of what makes us human.
Examiners expect you to name specific benefits relevant to human health: medical diagnosis, developing treatments for genetic diseases, personalised medicine, and identifying disease-causing genes. Avoid vague answers; be specific about how genome knowledge is applied.
Must Know
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Sexual reproduction produces genetically varied offspring (via meiosis, gamete fusion, and crossing over); asexual reproduction produces genetically identical clones (via mitosis). Sexual reproduction is slower but adaptable; asexual is fast but vulnerable to environmental change.
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DNA structure: Double helix of two antiparallel strands held together by complementary base pairing (A-T, C-G). Each nucleotide contains deoxyribose sugar, phosphate group, and a nitrogenous base; nucleotides link via the sugar-phosphate backbone.
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The genetic code: A triplet code where every three bases (codon) code for one amino acid. The code is degenerate (multiple codons code for the same amino acid) and universal across organisms. ATG is the start codon; UAA, UAG, UGA are stop codons.
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A gene is a sequence of DNA bases that codes for a specific sequence of amino acids in a protein.
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Protein synthesis: Transcription (RNA polymerase copies DNA to mRNA in the nucleus) → Translation (tRNA brings amino acids to ribosome, mRNA codons match tRNA anticodons, and amino acids are linked by peptide bonds). DNA → mRNA → Protein.
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Human Genome Project: Identified ~20,000–25,000 human genes and sequenced 3 billion base pairs. Benefits include identifying genes causing genetic diseases, developing gene therapies, enabling personalised medicine, and understanding human evolution.
That's the notes covered.
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